1. The Field of the Invention
The present invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to novel structures for a spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. The Relevant Technology
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads carrying read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are now the most common type of read sensors. This is largely due to the capability of MR heads of reading data on a disk of a greater linear density than that which the previously used thin film inductive heads are capable of. An MR sensor detects a magnetic field through a change in resistance in its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fexe2x80x94Mn) layer.
The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
FIG. 1 shows a typical prior art SV sensor 100 comprising a pair of end regions 104 and 106 separated by a central region 102. The central region 102 is formed by a suitable method such as sputtering and has defined end regions that are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 121.
The free layer 110, spacer layer 115, pinned layer 120 and AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I, from a current source 160 to the MR sensor 100. A sensing device 170 connected to the leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by an external magnetic field (e.g., a field generated by a data bit stored on a disk). IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
Another type of spin valve sensor recently developed is an antiparallel (AP)-pinned spin valve sensor. FIG. 2 shows one representative AP-pinned SV sensor 200. The AP-pinned SV sensor 200 has a pair of end regions 202 and 204 separated from each other by a central region 206. The AP-pinned SV sensor 200 is also shown comprising a Nixe2x80x94Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208 which is made of NiO.
The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 of cobalt and a second ferromagnetic layer 216 of cobalt separated from each other by a ruthenium (Ru) antiparallel coupling layer 214. The AFM layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. A pair of hard bias layers 235 and 240, formed in the end regions 202 and 204, provide longitudinal biasing for the free layer 225.
A pair of electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the SV sensor 200. In the depicted example, the magnetization direction of the free layer 225 is set parallel to the air bearing surface (ABS) in the absence of an external field. The magnetization directions of the pinned layers 212 and 216, respectively, are also set to be perpendicular to the ABS. The magnetization directions of the pinned layers are shown as coming out of the Figure at 260 and going in at 255. The magnetization of the free layer 225 is shown set to be parallel to the ABS.
The disk drive industry has been engaged in an ongoing effort to increase the overall sensitivity, or GMR coefficient, of the SV sensors in order to permit the drive head to read smaller changes in magnetic flux. Higher GMR coefficients enable the storage of more bits of information on any given disk surface. The GMR coefficient of an SV sensor is xcex94R/R, or the change in magnetoresistance of the sensor material, divided by the overall resistance of the material. The GMR coefficient is dependent on both the xe2x80x9csoftnessxe2x80x9d of the material and its overall resistance.
The softness of an SV sensor is a measurement of the threshold level of a magnetic field needed to change the magnetoresistance of a material by a given amount, typically that required to move the magnetic moment of the material from one orientation to another, offset from the first by 90xc2x0. The softness of a material is referred to as its coercivity, Hc, when the material exhibits domain walls, and more generally, by the property magnetocrystalline anisotropy, HK. The change in magnetoresistance, xcex94R, is proportional to the change in magnetic flux multiplied by the softness of the material. A high degree of softness is increasingly important as disk drive densities increase and the magnetic field strengths of the recorded materials correspondingly decrease.
A change in resistance of the sensor material can be easily measured only if the change is large compared to the overall resistance R of the material. Thus, a low overall resistance R, combined with a high change in magnetoresistance, xcex94R, will produce a high GMR coefficient.
Other properties relevant to the performance of a GMR head include magnetostriction, exchange coupling between the AFM and the pinned layer or layers, and the electrical resistivity of the AFM. Magnetostriction is a measure of the stress or deformation of a material when it undergoes a change in magnetism. It is desired in the construction of spin valves to keep magnetostriction to a minimum because deformation of the GMR head materials can cause poor interfacing between layers and nonlinear performance as magnetic flux changes.
Exchange coupling between the AFM and the pinned layers is important because magnetic flux from the AFM must reach the pinned layer with a minimum of reluctance or leakage in order to keep the magnetic moment of the pinned layer at a consistent orientation. An inadequate exchange coupling may cause poor pinning, thereby reducing the sensitivity of the GMR head.
It is also vital that the current through the spin valve sensor be confined to the pinned and sensing portions of the spin valve sensor. If current is permitted to shunt through the AFM layer, the magnetoresistance recorded by the sensor will be artificially low, thus producing a lower GMR coefficient and a nonlinear signal. Thus, the material selected for the AFM layer must possess a high electrical resistivity in order to prevent shunting.
Drive heads have been produced by forming a seed or buffer layer on or near the substrate, and then forming the remaining layers on top of the seed layer. The crystalline structure and orientation of the seed layer determine the configuration of the remaining layers. Materials such as NiFe have previously been used to form all or part of the seed layer.
Unfortunately, bottom layers used in the past tend to form along uneven crystallographic planes, sometimes even producing a jagged interface with a mixture of nonparallel planes. Consequently, the atoms that form the boundary of the seed layer are spread far apart, and layers formed above the seed layer exhibit an uneven texture and weak bonding. The result is a lower magnetic softness and poor exchange coupling between the AFM and the pinned layer or layers.
It is believed by the inventor that providing a seed layer with a denser and more homogeneous boundary would promote even-textured growth of the layers above it, as well as tight interfaces between adjacent layers. Thus, the GMR coefficient of the spin valve sensor would be enhanced by the higher softness and lower resistance to current movement between the sensing layers. In addition, the exchange coupling between the AFM and the pinned layer or layers would be improved for a further boost in magnetic sensitivity.
The selection of structures used to produce the pinned layer or layers is also believed to be critical. NiFe is also used in many prior art pinned layers. Many currently used pinned layers provide no significant barrier to electrical current. Thus, the electrical resistance of materials such as NiFe is generally inadequate to prevent current shunting. As a result, current that should encounter higher resistance as it travels through the sensing layer and the pinned layer or layers is permitted to take a xe2x80x9cshortcutxe2x80x9d through the AFM. Accordingly, xcex94R decreases and causes a corresponding decline in the GMR coefficient.
Therefore, It would also be desirable to form a layer above the AFM that has a high electrical resistance. Such a layer would prevent current shunting and ensure that xcex94R remains highly sensitive to changes in magnetic flux, thus further improving the GMR coefficient.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available spin valve sensors. Thus, it is an overall objective of the present invention to provide an improved spin valve sensor that overcomes some or all of the problems discussed above as existing in the art.
To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, a spin valve sensor is provided and configured with a bottom structure enhanced through the addition of one or more crystallographically aligned, electrically resistive layers.
The spin valve sensor of the present invention in one embodiment comprises a seed layer including a layer of NiFeNb and a layer of metal oxide, such as NiO. In alternate embodiments of the invention, a layer of a heavy metal, such as Ta, may be sandwiched between the NiFeNb and metal oxide layers, as part of the seed layer.
Selected embodiments provide a layer of NiFeNb within the pinned layer. In this configuration, the NiFeNb layer acts as a high resistivity buffer layer for layers underneath. This high resistivity buffer layer may be provided with or without the use of NiFeNb in the seed layer.
The spin valve sensor may comprise a cap layer, a free layer, a spacer layer, a pinned layer of ferromagnetic material as discussed above, an antiferromagnetic (AFM) layer, and a substrate. Nevertheless, the seed and high resistivity buffer layers of the present invention are intended for use with any type of spin valve sensor having a suitable construction.
The spin valve sensor of the present invention may be incorporated within a disk drive system comprising a magnetic recording disk; an anti-parallel (AP)-pinned spin valve (SV) sensor configured in the manner discussed above; an actuator for moving the spin valve sensor across the magnetic recording disk so the spin valve sensor may access different regions of magnetically recorded data on the magnetic recording disk; and a detector electrically coupled to the spin valve sensor for detecting changes in resistance of the sensor caused by rotation of the magnetization axis of the free ferromagnetic layer relative to the fixed magnetizations of the AP-pinned layer in response to magnetic fields from the magnetically recorded data.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.